Aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid are used to produce a variety of polyester products, important examples of which are poly (ethylene terephthalate) and its copolymers. These aromatic dicarboxylic acids are synthesized by the catalyzed autoxidation of the corresponding dialkyl aromatic compounds which are obtained from fossil fuels (US 2006/0205977 A1). There is a growing interest in the use of renewable resources as feed stocks for the chemical industries mainly due to the progressive reduction of fossil reserves and their related environmental impacts.
Furan 2,5-dicarboxylic acid (“FDCA”) is a versatile intermediate considered as a promising closest biobased alternative to terephthalic acid and isophthalic acid. It is synthesized by the catalytic oxidation of 5-(hydroxymethyl)furfural (5-HMF) as shown in equation 1 below; or by the catalytic oxidation of 5-HMF esters (5-R(CO)OCH2-furfural where R=alkyl, cycloalkyl and aryl) as shown in equation 2 below; or by the catalytic oxidation of 5-HMF ethers (5-R′OCH2-furfural, where R′=alkyl, cycloalkyl and aryl) as shown in equation 3 below; or by the catalytic oxidation of 5-alkyl furfurals (5-R″-furfural, where R″=alkyl, cycloalkyl and aryl) as shown in equation 4 below; in each case using a Co/Mn/Br catalyst system. Mixed feedstocks of 5-HMF and 5-HMF esters, mixed feedstocks of 5-HMF and 5-HMF ethers, and mixed feedstocks of 5-HMF and 5-alkyl furfurals can also be used.

We have found that the above reactions work well. However a number of impurities are produced, particularly mono-carboxylic acid species such as 5-formyl furan-2-carboxylic acid (FFCA). These mono-carboxylic acids are not desirable since they terminate the chain growth of a polymer resulting in lower polymer viscosity. If colored bodies are present in the crude FDCA or remaining in the purified FDCA, these colored bodies carry through to compounds or polymers using the FDCA as a reactive monomer to thereby color the compound or polymer. Therefore, it is necessary to purify the crude FDCA to remove the color bodies while minimizing the presence of FFCA in the purified FDCA.
FDCA has been prepared by oxidation of 5-(hydroxymethyl) furfural (5-HMF) under air using homogenous catalysts (US2003/0055271 A1 and Partenheimer, W.; Grushin, V. V. Adv. Synth. Catal. 2001, 343, 102-111.) but only a maximum of 44.8% yield using Co/Mn/Br catalysts system and a maximum of 60.9% yield was reported using Co/Mn/Br/Zr catalysts combination. Heterogeneous catalysis oxidation of 5-HMF using ZrO2 mixed with platinum (II) acetylacetonate in water has been reported in U.S. Pat. No. 7,700,788 B2, but due to very low solubility of FDCA in water, this process needs to be conducted under very dilute conditions to avoid precipitation of FDCA on the catalysts surface which makes the process not economical. Another heterogeneous catalysis oxidation of 5-HMF is reported in U.S. Pat. No. 4,977,283 using molecular O2 and a Pt/C catalyst. High FDCA yield was achieved but at the extra expense of feeding purified O2 and continually adjusting pH via sodium hydroxide addition. The reaction product was the disodium salt of FDCA leading to a wasteful salt by-product in the conversion to FDCA.
There remains a need to produce a FDCA at high yields and isolate purified FDCA product that has low color.